Traces of antimatter in cosmic rays revive the search for “WIMPs” as dark matter

One of the great challenges of modern cosmology is to reveal the nature of dark matter. We know it exists (it makes up more than 85% of the matter in the universe), but we have never seen it directly and still don’t know what it is.

A study published in the Journal of Cosmology and Astroparticle Physics examined traces of antimatter in the cosmos that could reveal a new class of particles never before observed, called WIMPs (Weakly Interacting Massive Particles), which could constitute dark matter.

The study suggests that some recent observations of “antinuclei” in cosmic rays are consistent with the existence of WIMPs, but also that these particles may be even stranger than previously thought.

“WIMPs are particles theorized but never observed, and they could be ideal candidates for dark matter,” explains Pedro De la Torre Luque, a physicist at the Institute of Theoretical Physics in Madrid.

A few years ago, the scientific community hailed it as a “miracle”. WIMPs seemed to meet all the requirements for dark matter, and it was thought – once we had “imagined” what they could be and how they could be detected – that within a few years we would have the first proof direct of their existence.

On the contrary, research carried out in recent years has led to the exclusion of entire classes of these particles, due to their particular emissions. Today, although their existence has not been completely ruled out, the range of possible types of WIMP has been considerably reduced, as have the methodologies for attempting to detect them.

“Of the many best-motivated proposed models, most have been discarded today and only a few of them survive today,” says De la Torre Luque.

A recent discovery, however, seems to have reopened the case. “These are some observations from the AMS-02 experiment,” explains De la Torre Luque. AMS-02 (Alpha Magnetic Spectrometer) is a scientific experiment aboard the International Space Station that studies cosmic rays. “Project leaders revealed that they had detected traces of antinuclei in cosmic rays, including antihelium, which no one expected.”

To understand why these antinuclei are important to WIMPs and dark matter, we first need to understand what antimatter is.

Antimatter is a form of matter with an electrical charge opposite that of “normal” matter particles. Ordinary matter consists of particles with a negative electrical charge, such as electrons, a positive charge (protons) or a neutral charge.

Antimatter is composed of “mirror” particles with opposite charges (a “positive” electron, the positron, a “negative” proton, etc.). When matter and antimatter meet, they annihilate each other by emitting powerful gamma radiation.

In the universe, composed overwhelmingly of normal matter, there is a small amount of antimatter, sometimes closer than one might think, given that positrons are used as contrast agents for PET, medical imaging exam that some of you may have undergone.

Some of this antimatter was formed – according to scientists – during the Big Bang, but more is constantly being created by specific events, making it very important to observe. “If you see the production of antiparticles in the interstellar medium, where you would expect very few, that means something unusual is happening,” explains De la Torre Luque. “That’s why observing antihelium was so exciting.”

What produces the antihelium nuclei observed by AMS-02 could indeed be WIMPs. According to this theory, when two WIMP particles meet, in some cases they annihilate each other, meaning they destroy each other, emitting energy and producing both matter and antimatter particles.

De la Torre Luque and his colleagues tested some WIMP models to see if they were consistent with the observations.

The study confirmed that some antihelium observations are difficult to explain by known astrophysical phenomena.

“Theoretical predictions suggest that, although cosmic rays can produce antiparticles through interaction with gases in the interstellar medium, the amount of antinuclei, particularly antihelium, should be extremely small,” explains De la Torre Luque.

“We expected to detect an antihelium event every few decades, but the dozen or so antihelium events observed by AMS-02 are orders of magnitude greater than predictions based on standard cosmic ray interactions. . This is why these antinuclei are a plausible clue to the annihilation of WIMPs.

But there may be more. The antihelium nuclei observed by AMS-02 consist of two distinct isotopes (the same element, but with varying numbers of neutrons in the nucleus), antihelium-3 and antihelium-4. Antihelium-4, in particular, is much heavier and also much rarer.

We know that the production of heavier nuclei becomes increasingly unlikely as their mass increases, in part because of natural processes involving cosmic rays. This is why seeing such a large number is a warning sign.

“Even in the most optimistic models, WIMPs could only explain the amount of antihelium-3 detected, but not antihelium-4,” continues De la Torre Luque, and this would require imagining a particle (or a class of particles) even stranger than the WIMPs proposed so far, or in technical jargon, even more “exotic”.

Thus, the study by De la Torre Luque and colleagues indicates that the path to WIMPs is not yet closed. Many more precise observations are now needed, and we may need to expand or adapt the theoretical model, perhaps introducing a new dark sector into the standard model of particles known to date, with new “exotic” elements.